Asymmetric Orbifolds, Non-Geometric Fluxes and Non-Commutativity in Closed String Theory

In this paper we consider a class of exactly solvable closed string flux backgrounds that exhibit non-commutativity in the closed string coordinates. They are realized in terms of freely-acting asymmetric Z_N-orbifolds, which are themselves close relatives of twisted torus fibrations with elliptic Z_N-monodromy (elliptic T-folds). We explicitly construct the modular invariant partition function of the models and derive the non-commutative algebra in the string coordinates, which is exact to all orders in {\alpha}'. Finally, we relate these asymmetric orbifold spaces to inherently stringy Scherk-Schwarz backgrounds and non-geometric fluxes.Comment: 30 page

Massless D-strings and moduli stabilization in type I cosmology

We consider the cosmological evolution induced by the free energy F of a gas of maximally supersymmetric heterotic strings at finite temperature and weak coupling in dimension D>=4. We show that F, which plays the role of an effective potential, has minima associated to enhanced gauge symmetries, where all internal moduli can be attracted and dynamically stabilized. Using the fact that the heterotic/type I S-duality remains valid at finite temperature and can be applied at each instant of a quasi-static evolution, we find in the dual type I cosmology that all internal NS-NS and RR moduli in the closed string sector and the Wilson lines in the open string sector can be stabilized. For the special case of D=6, the internal volume modulus remains a flat direction, while the dilaton is stabilized. An essential role is played by light D-string modes wrapping the internal manifold and whose contribution to the free energy cannot be omitted, even when the type I string is at weak coupling. As a result, the order of magnitude of the internal radii expectation values on the type I side is (lambda_I alpha')^{1/2}, where lambda_I is the ten-dimensional string coupling. The non-perturbative corrections to the type I free energy can alternatively be described as effects of "thermal E1-instantons", whose worldsheets wrap the compact Euclidean time cycle.Comment: 39 pages, 1 figur

Price adjustment method and ex-dividend day returns in a different institutional setting

This article was published in the International Review of Financial Analysis [© Elsevier Inc.] and the definitive version is available at: http://dx.doi.org/10.1016/j.irfa.2015.05.005This study investigates the determinants of the ex-dividend day price behavior in the Athens Stock Exchange (ASE), a unique institutional setting, and examines how a major regulatory change in the price adjustment method affects the extent of the ex-day stock price drop. We find that allowing the market to freely adjust prices, after 2001, the ex-dividend day price improves the pricing efficiency of the market in the sense that the raw price ratio tends to one and abnormal returns tend to zero. We also find that in the absence of taxes on dividends and capital gains and certain microstructure impediments discussed in the literature - i.e., bid-ask spread, market makers, price discreteness, tick size and limit order adjustment mechanism - stock illiquidity is the best candidate for explaining the magnitude of the ex-dividend day price adjustment